Method and apparatus for controlling fume hood face velocity using variable by-pass resistance

ABSTRACT

A fume hood controller and method which compensates for resistance to air flow through a by-pass opening that exists due to a grille or louvre overlying the by-pass opening.

The present invention relates generally to the control of theventilation of laboratory fume hoods, and more particularly to animproved method and apparatus for controlling the open face velocity ofa fume hood that has a by-pass area that has a grille or louverstructure overlying the by-pass area, and which by-pass area is at leastpartially covered by opening the fume hood doors.

Fume hoods are utilized in various laboratory environments for providinga work place where potentially dangerous chemicals are used, with thehoods comprising an enclosure having moveable doors at the front portionthereof which can be opened in various amounts to permit a person togain access to the interior of the enclosure for the purpose ofconducting experiments and the like. The enclosure is typicallyconnected to an exhaust system for removing any noxious fumes so thatthe person will not be exposed to them while performing work in thehood.

Fume hood controllers which control the flow of air through theenclosure have become more sophisticated in recent years, and are nowable to more accurately maintain the desired flow characteristics toefficiently exhaust the fumes from the enclosure as a function of thedesired average face velocity of the opening of the fume hood. Theaverage face velocity is generally defined as the flow of air into thefume hood per square foot of open face area of the fume hood, with thesize of the open face area being dependent upon the position of one ormore moveable doors that are provided on the front of the enclosure orfume hood, and in most types of enclosures, the amount of by-passopening that is provided when the door or doors are closed.

The fume hoods are exhausted by an exhaust system that includes a blowerthat is capable of being driven at variable speeds to increase ordecrease the flow of air from the fume hood to compensate for thevarying size of the opening or face. Alternatively, there may be asingle blower connected to the exhaust manifold that is in turnconnected to the individual ducts of multiple fume hoods, and dampersmay be provided in the individual ducts to control the flow from theindividual ducts to thereby modulate the flow to maintain the desiredaverage face velocity.

The doors of such fume hoods can be opened by raising them vertically,often referred to as the sash position, or some fume hoods have a numberof doors that are mounted for sliding movement in typically two sets ofvertical tracks. There are even doors that can be moved horizontally andvertically, with the tracks being mounted in a frame assembly that isvertically movable.

Very recent improvements in controlling the operation of fume hoods havebeen made and are disclosed in the following patents and applicationsthat have been assigned to the same assignee as the present invention:Apparatus for Controlling the Ventilation of a Laboratory Fume Hood byAhmed et. al, Ser. No.: 52370; Apparatus for Determining the Position ofa Moveable Structure Along a Track by Egbers et. al, Ser. No.: 52496; ASystem for Controlling the Differential Pressure of a Room HavingLaboratory Fume Hoods by Ahmed et. al, Ser. No. 52497; A Method andApparatus for Determining the Uncovered Size of an Opening Adapted to beCovered by Multiple Moveable Doors, by Ahmed et. al, Ser. No. 52498; andLaboratory Fume Hood Control Apparatus Having Improved SafetyConsiderations, by Ahmed, Ser. No.

While the above referenced patents and applications are directed toimproved controllers for fume hoods, the existence of grilles or baffleswhich cover those types of fume hoods which have by-pass areas have beenfound to affect the operational control of such fume hoods. Because thepresence of such grilles or baffles necessarily affect the flow of airthrough the grilles, the use of the calculated overall area of theby-pass area alone may not provide an accurate parameter in which tocontrol the flow of air through the area that is uncovered by the doorsof the fume hood.

Accordingly, it is a primary object of the present invention to providean improved fume hood controller and method for accurately controllingthe flow of air through the uncovered area of the fume hood having aby-pass, and for compensating for the presence of grilles or baffleswhich may cover the by-pass area.

It is another object of the present invention to provide such animproved fume hood controller that is easily adaptable for use incontrolling most commercially available fume hoods of the type in whichchanging the position of the sash doors changes the effective size ofthe by-pass area, and accurately controls the flow of air through theuncovered opening to the fume hood, taking into consideration the numberof sash doors, the sizes of the sash doors and any by-pass area whichmay have a grille or baffle overlying the by-pass area.

A more specific object of the present invention is to provide such acontroller and method which utilizes a conductance factor to compensatefor any resistance to flow of air through a baffle or grille covering aby-pass area, so that more accurate control of the fume hood can beachieved.

These and other objects will become apparent upon reading the followingdetailed description of the present invention, while referring to theattached drawings, in which:

FIG. 1 is a schematic block diagram of apparatus of the presentinvention shown integrated with a room controller of a heating,ventilating and air conditioning monitoring and control system of abuilding;

FIG. 2 is a block diagram of a fume hood controller, shown connected toan operator panel, the latter being shown in front elevation;

FIG. 3 is a diagrammatic elevation of the front of a representative fumehood having a vertically operable sash door, and a by-pass openinglocated above the front face, with the by-pass opening having a grillecovering the same;

FIG. 4 is a diagrammatic elevation of the front of a representative fumehood having horizontally operable sash doors;

FIG. 5 is a cross section taken generally along the line 5--5 of FIG. 4;

FIG. 6 is a diagrammatic elevation of the front of a representativecombination sash fume hood having horizontally and vertically operablesash doors;

FIG. 7 is an electrical schematic diagram of a plurality of door sashposition indicating switching means;

FIG. 8 is a cross section of the door sash position switching means;

FIG. 9 is a schematic diagram of electrical circuitry for determiningthe position of sash doors of a fume hood;

FIG. 10 is a block diagram illustrating the relative positions of FIGS.10a, 10b, 10c, 10d and 10e to one another, and which together comprise aschematic diagram of the electrical circuitry for the fume hoodcontroller means embodying the present invention;

FIGS. 10a, 10b, 10c, 10d and 10e, which if connected together, comprisethe schematic diagram of the electrical circuitry for the fume hoodcontroller means embodying the present invention;

FIG. 11 is a flow chart of the general operation of the fume hoodcontroller;

FIG. 12 is a flow chart of a portion of the operation of the fume hoodcontroller, particularly illustrating the operation of the feed forwardcontrol scheme, which may be employed;

FIG. 13 is a flow chart of a portion of the operation of the fume hoodcontroller, particularly illustrating the operation of the proportionalgain, integral gain and derivative gain control schemes;

FIG. 14 is a flow chart of a portion of the operation of the fume hoodcontroller, particularly illustrating the operation of the calibrationof the feed forward control scheme;

FIG. 15 is a flow chart of a portion of the operation of the fume hoodcontroller embodying the present invention, particularly illustratingthe operation of the calculation of the uncovered opening for a numberof horizontally moveable sash doors; and,

FIG. 16 is a flow chart of a portion of the operation of the fume hoodcontroller embodying the present invention, particularly illustratingthe operation of the calculation of the uncovered opening for a numberof horizontally and vertically moveable sash doors; and,

FIG. 17 is a flow chart of a portion of the operation of the fume hoodcontroller embodying the present invention, particularly illustratingthe operation of the calculation of the uncovered opening for avertically moveable sash door in a fume hood having a by-pass openingwith a grille overlying the same.

DETAILED DESCRIPTION

It should be generally understood that a fume hood controller controlsthe flow of air through the fume hood in a manner whereby the effectivesize of the total opening to the fume hood, including the portion of theopening that is not covered by one or more sash doors will have arelatively constant average face velocity of air moving into the fumehood. This means that regardless of the area of the uncovered opening,an average volume of air per unit of surface area of the uncoveredportion will be moved into the fume hood. This protects the persons inthe laboratory from being exposed to noxious fumes or the like becauseair is always flowing into the fume hood, and out of the exhaust duct,and the flow is preferably controlled at a predetermined rate ofapproximately 75 to 150 cubic feet per minute per square feet ofeffective surface area of the uncovered opening. In other words, if thesash door or doors are moved to the maximum open position whereby anoperator has the maximum access to the inside of the fume hood forconducting experiments or the like, then the flow of air will mostlikely have to be increased to maintain the average face velocity at thepredetermined desired level.

Some fume hoods have a by-pass opening through which air is moved andexpelled through the exhaust duct even when the sash doors are closed.Moreover, the effective size of the by-pass opening is often reducedwhen the sash doors are opened, particularly when the sash doors areopened vertically, and the by-pass opening is located above the sashdoors. For aesthetic purposes, a grille or baffle is often located tocover the by-pass opening, and such a grille necessarily provides someresistance to air flow through the by-pass opening.

Broadly stated, the present invention is directed to an improved fumehood controlling apparatus that is adapted to provide many desirableoperational advantages for persons who use the fume hoods to performexperiments or other work, and also for the operator of the facility inwhich the fume hoods are located. The apparatus embodying the presentinvention provides extremely rapid, accurate and effective control ofthe average face velocity of the fume hood, and achieves and maintainsthe desired average face velocity within a few seconds after one or moredoors which cover the front opening of the fume hood have been moved.This is achieved, at least in part, by the rapid calculation of theuncovered area of the opening of the fume hood, i.e., that area notcovered by sash doors, frames, lips and the like, which calculation isrepeated several times per second. The fume hood controller apparatusembodying the present invention includes a computing means, togetherwith associated memory, which can be configured for horizontally and/orvertically moveable sash doors by inputting the necessary dimensions ofthe sash doors and other structural features, such as the upper lipheight, frame widths and the like, as will be described.

Additionally, the determination of the volume of air that must be drawnthrough the exhaust duct by controlling the position of a damper or thespeed of a blower that is necessary to provide a predetermined averageface velocity through the uncovered area of the opening of the fume hoodmust take into consideration the volume of air that is being drawnthrough a by-pass opening if one is provided. It should be appreciatedthat any calculation of the total volume that is necessary to providethe desired average face velocity if based merely on the size of theuncovered opening and the uncovered by-pass opening may result ininaccuracies if the by-pass opening has a grille overlying the by-passopening, because the grille may provide some resistance to air flow thatwould otherwise occur if the grille were not present.

Turning now to the drawings, and particularly FIG. 1, a block diagram isshown of several fume hood controllers 20 interconnected with a roomcontroller 22, an exhaust controller 24 and a main control console 26.The fume hood controllers 20 are interconnected with the room controller22 and with the exhaust controller 24 and the main control console 26 ina local area network illustrated by line 28 which may be amulticonductor cable or the like. The room controller, the exhaustcontroller 24 and the main control console 26 are typically part of thebuilding main HVAC system in which the laboratory rooms containing thefume hoods are located. The fume hood controllers 20 are provided withpower through line 30, which is at the proper voltage via a transformer32 or the like.

The room controller 22 preferably is of the type which is at leastcapable of providing a variable air volume to the room, and may be aLandis & Gyr Powers System 600 SCU controller. The room controller 22 iscapable of communicating over the LAN lines 28. The room controllerpreferably is a System 600 SCU controller and is a commerciallyavailable controller for which extensive documentation exists. The UserReference Manual, Part No. 125-1753 for the System 600 SCU controller isspecifically incorporated by reference herein.

The room controller 22 receives signals via lines 81 from each of thefume hood controllers 20 that provides an analog input signal indicatingthe volume of air that is being exhausted by each of the fume hoodcontrollers 20 and a comparable signal from the exhaust flow sensor thatprovides an indication of the volume of air that is being exhaustedthrough the main exhaust system apart from the fume hood exhausts. Thesesignals coupled with signals that are supplied by a differentialpressure sensor 29 which indicates the pressure within the room relativeto the reference space enable the room controller to control the supplyof air that is necessary to maintain the differential pressure withinthe room at a slightly lower pressure than the reference space, i.e.,preferably within the range of about 0.05 to about 0.1 inches of water,which results in the desirable lower pressure of the room relative tothe reference space. However, it is not so low that it prevents personsinside the laboratory room from opening the doors to escape in the eventof an emergency, particularly if the doors open outwardly from the room.Also, in the event the doors open inwardly, the differential pressurewill not be so great that it will pull the door open due to excessiveforce being applied due to such pressure.

The sensor 29 is preferably positioned in a suitable hole or opening inthe wall between the room and the reference space and measures thepressure on one side relative to the other. Alternatively, a velocitysensor may be provided which measures the velocity of air moving throughthe opening which is directly proportional to the pressure differencebetween the two spaces. Of course, a lower pressure in the room relativeto the reference space would mean that air would be moving into the roomwhich is also capable of being detected.

Referring to FIG. 2, a fume hood controller 20 is illustrated with itsinput and output connector ports being identified, and the fume hoodcontroller 20 is connected to an operator panel 34. It should beunderstood that each fume hood will have a fume hood controller 20 andthat an operator panel will be provided with each fume hood controller.The operator panel 34 is provided for each of the fume hoods and it isinterconnected with the fume hood controller 20 by a line 36 whichpreferably comprises a multi-conductor cable having eight conductors.The operator panel has a connector 38, such as a 6 wire RJ11 typetelephone jack for example, into which a lap top personal computer orthe like may be connected for the purpose of inputting informationrelating to the configuration or operation of the fume hood duringinitial installation, or to change certain operating parameters ifnecessary. The operator panel 34 is preferably mounted to the fume hoodin a convenient location adapted to be easily observed by a person whois working with the fume hood.

The fume hood controller operator panel 34 includes a liquid crystaldisplay 40 which when selectively activated provides the visualindication of various aspects of the operation of the fume hood,including three digits 42 which provide the average face velocity. Thedisplay 40 illustrates other conditions such as low face velocity, highface velocity and emergency condition and an indication of controllerfailure. The operator panel may have an alarm 44 and an emergency purgeswitch 46 which an operator can press to purge the fume hood in theevent of an accident. The operator panel has two auxiliary switches 48which can be used for various customer needs, including day/night modesof operation. It is contemplated that night time mode of operation wouldhave a different and preferably reduced average face velocity,presumably because no one would be working in the area and such a loweraverage face velocity would conserve energy. An alarm silence switch 50is also preferably provided to extinguish an alarm.

Fume hoods come in many different styles, sizes and configurations,including those which have a single sash door or a number of sash doors,with the sash doors being moveable vertically, horizontally or in bothdirections. Additionally, various fume hoods have different amounts ofby-pass flow, i.e., the amount of flow permitting opening that existseven when all of the sash doors are as completely closed as their designpermits. Other design considerations involve whether there is some kindof filtering means included in the fume hood for confining fumes withinthe hood during operation. While many of these design considerationsmust be taken into account in providing efficient and effective controlof the fume hoods, the apparatus can be configured to account forvirtually all of the above described design variables, and effective andextremely fast control of the fume hood ventilation is provided.

Referring to FIG. 3, there is shown a fume hood, indicated generally at60, which has a vertically operated sash door 62 which can be moved togain access to the fume hood and which can be moved to the substantiallyclosed position as shown. Fume hoods are generally designed so that evenwhen a door sash such as door sash 62 is completely closed, there isstill some amount of opening into the fume hood, such as opening 63,through which air can pass. This opening is generally referred to as theby-pass area and it can be determined so that its effect can be takeninto consideration in controlling the flow of air into the fume hood.Some types of fume hoods have a by-pass opening that is located abovethe door sash while others are below the same. In some fume hoods, thefirst amount of movement of a sash door will increase the opening at thebottom of the door shown in FIG. 3, for example, but as the door israised, it will merely cut off the by-pass opening so that the size ofthe total opening of the fume hood is maintained relatively constant forperhaps the first one-fourth amount of movement of the sash door 62through its course of travel and ignoring any effect of a grille 65which is provided to overlie the by-pass area.

Other types of fume hoods may include several horizontally moveable sashdoors 66 such as shown in FIGS. 4 and 5, with the doors being movable inupper and lower pairs of adjacent tracks 68. When the doors arepositioned as shown in FIGS. 4 and 5, the fume hood opening iscompletely closed and an operator may move the doors in the horizontaldirection to gain access to the fume hood. Both of the fumes hoods 60and 64 have an exhaust duct 70 which generally extends to an exhaustsystem which may be that of the HVAC apparatus previously described. Thefume hood 64 also includes a filtering structure shown diagrammaticallyat 72, which filtering structure is intended to keep noxious fumes andother contaminants from exiting the fume hood into the exhaust system.

Referring to FIG. 6, there is shown a combination fume hood which hashorizontally movable doors 76 which are similar to the doors 66, withthe fume hood 74 having a frame structure 78 which carries the doors 76in suitable tracks and the frame structure 78 is also vertically movablein the opening of the fume hood.

The illustration of FIG. 6 has portions removed as shown by the breaklines 73 which is intended to illustrate that the height of the fumehood may be greater than is otherwise shown so that the frame structure78 may be raised sufficiently to permit adequate access to the interiorof the fume hood by a person. There is generally a by-pass area which isidentified as the vertical area 75, and there is typically a top lipportion 77 which may be approximately 2 inches wide. This dimension ispreferably defined so that its effect on the calculation of the openface area can be taken into consideration.

While not specifically illustrated, other combinations are alsopossible, including multiple sets of vertically moveable sash doorspositioned adjacent one another along the width of the fume hoodopening, with two or more sash doors being vertically moveable inadjacent tracks, much the same as residential casement windows.

In accordance with an important aspect of the fume hood controller 20,it is adapted to operate the fume hoods of various sizes andconfigurations as has been described, and it is also adapted to beincorporated into a laboratory room where several fume hoods may belocated and which may have exhaust ducts which merge into a commonexhaust manifold which may be a part of the building HVAC system. A fumehood may be a single self-contained installation and may have its ownseparate exhaust duct. In the event that a single fume hood isinstalled, it is typical that such an installation would have a variablespeed motor driven blower associated with the exhaust duct whereby thespeed of the motor and blower can be variably controlled to therebyadjust the flow of air through the fume hood. Alternatively, and mosttypically for multiple fume hoods in a single area, the exhaust ducts ofeach fume hood are merged into one or more larger exhaust manifolds anda single large blower may be provided in the manifold system. In suchtypes of installations, control of each fume hood is achieved by meansof separate dampers located in the exhaust duct of each fume hood, sothat variation in the flow can be controlled by appropriatelypositioning the damper associated with each fume hood.

The fume hood controller is adapted to control virtually any of thevarious kinds and styles of fume hoods that are commercially available,and to this end, it has a number of input and output ports (lines,connectors or connections, all considered to be equivalent herein) thatcan be connected to various sensors that may be used with thecontroller. As shown in FIG. 2, it has digital output or DO ports whichinterface with a digital signal/analog pressure transducer with anexhaust damper as previously described, but it also has an analogvoltage output port for controlling a variable speed fan drive if it isto be installed in that manner. There are five sash position sensorports for use in sensing the position of both horizontally andvertically moveable sashes and there is also an analog input portprovided for connection to an exhaust air flow sensor 49. A digitalinput port for the emergency switch is provided and digital output portsfor outputting an alarm horn signal as well as an auxiliary signal isprovided. An analog voltage output port is also provided for providing avolume of flow signal to the room controller 22. In certain applicationswhere the exhaust air flow sensor is not provided, a wall velocitysensor indicative of face velocity may be utilized and an input port forsuch a signal is provided, but the use of such sensors is generallyconsidered to be less accurate and is not the preferred embodiment. Withthese various input and output ports, virtually any type of fume hoodcan be controlled in an effective and efficient manner.

From the foregoing discussion, it should be appreciated that if thedesired average face velocity is desired to be maintained and the sashposition is changed, the size of the opening can be dramatically changedwhich may then require a dramatic change in the volume of air tomaintain the average face velocity. While it is known to control avariable air volume blower as a function of the sash position, the fumehood controller apparatus improves on that known method by incorporatingadditional control schemes which dramatically improve the capabilitiesof the control system in terms of maintaining relatively constantaverage face velocity in a manner whereby reactions to perturbations inthe system are quickly made.

To determine the position of the sash doors, a sash position sensor isprovided adjacent each movable sash door and it is generally illustratedin FIGS. 7, 8 and 9. Referring to FIG. 8, the door sash positionindicator comprises an elongated switch mechanism 80 of relativelysimple mechanical design which preferably consists of a relatively thinpolyester base layer 82 upon which is printed a strip of electricallyresistive ink 84 of a known constant resistance per unit length. Anotherpolyester base layer 86 is provided and it has a strip of electricallyconductive ink 88 printed on it. The two base layers 82 and 86 areadhesively bonded to one another by two beads of adhesive 90 located onopposite sides of the strip. The base layers are preferablyapproximately five-thousandths of an inch thick and the beads areapproximately two-thousandths of an inch thick, with the beads providinga spaced area between the conductive and resistive layers 88 and 84. Theswitching mechanism 80 is preferably applied to the fume hood by a layerof adhesive 92.

The polyester material is sufficiently flexible to enable one layer tobe moved toward the other so that contact is made in response to apreferably spring biased actuator 94 carried by the appropriate sashdoor to which the strip is placed adjacent to so that when the sash dooris moved, the actuator 94 moves along the switching mechanism 80 andprovides contact between the resistive and conductive layers which arethen sensed by electrical circuitry to be described which provides avoltage output that is indicative of the position of the actuator 94along the length of the switching means. Stated in other words, theactuator 94 is carried by the door and therefore provides an electricalvoltage that is indicative of the position of the sash door.

The actuator 94 is preferably spring biased toward the switchingmechanism 80 so that as the door is moved, sufficient pressure isapplied to the switching means to bring the two base layers together sothat the resistive and conductive layers make electrical contact withone another and if this is done, the voltage level is provided. Byhaving the switching means 80 of sufficient length so that the fullextent of the travel of the sash door is provided as shown in FIG. 3,then an accurate determination of the sash position can be made.

It should be understood that the illustration of the switching mechanism80 in FIGS. 3 and 5 is intended to be diagrammatic, in that theswitching mechanism is preferably actually located within the sash frameitself and accordingly would not be visible as shown. The width andthickness dimensions of the switching mechanism are so small thatinterference with the operation of the sash door is virtually noproblem. The actuator 94 can also be placed in a small hole that may bedrilled in the sash door or it may be attached externally at one endthereof so that it can be in position to operate the switch 80. In thevertical moveable sash doors shown in FIGS. 3 and 6, a switchingmechanism 80 is preferably provided in one or the other of the sides ofthe sash frame, whereas in the fume hoods having horizontally movabledoors, it is preferred that the switching mechanism 80 be placed in thetop of the tracks 68 so that the weight of the movable doors do notoperate the switching mechanism 80 or otherwise damage the same.

Turning to FIG. 9, the preferred electrical circuitry which generatesthe position indicating voltage is illustrated, and this circuitry isadapted to provide two separate voltages indicating the position of twosash doors in a single track. With respect to the cross-section shown inFIG. 5, there are two horizontal tracks, each of which carries two sashdoors and a switching mechanism 80 is provided for each of the tracks asis a circuit as shown in FIG. 9, thereby providing a distinct voltagefor each of the four sash doors as shown.

The switching means is preferably applied to the fume hood with a layerof adhesive 92 and the actuator 94 is adapted to bear upon the switchingmeans at locations along the length thereof. Referring to FIG. 7, adiagrammatic illustration of a pair of switching means is illustratedsuch as may occur with respect to the two tracks shown in FIG. 5. Aswitching mechanism 80 is provided with each track and the four arrowsillustrated represent the point of contact created by the actuators 94which result in a signal being applied on each of the ends of eachswitching means, with the magnitude of the signal representing a voltagethat is proportional to the distance between the end and the nearestarrow. Thus, a single switching mechanism 80 is adapted to provideposition indicating signals for two doors located in each track. Thecircuitry that is used to accomplish the voltage generation is shown inFIG. 9 and includes one of these circuits for each track. The resistiveelement is shown at 84 and the conductive element 88 is also illustratedbeing connected to ground with two arrows being illustrated, andrepresented the point of contact between the resistive and conductiveelements caused by each of the actuators 94 associated with the twoseparate doors. The circuitry includes an operational amplifier 100which has its output connected to the base of a PNP transistor 102, theemitter of which is connected to a source of positive voltage throughresistor 104 into the negative input of the operational amplifier, thepositive input of which is also connected to a source of positivevoltage of preferably approximately five volts. The collector of thetransistor 102 is connected to one end of the resistive element 84 andhas an output line 106 on which the voltage is produced that isindicative of the position of the door.

The circuit operates to provide a constant current directed into theresistive element 84 and this current results in a voltage on line 106that is proportional to the resistance value between the collector andground which changes as the nearest point of contact along theresistance changes. The operational amplifier operates to attempt todrive the negative input to equal the voltage level on the positiveinput and this results in the current applied at the output of theoperational amplifier varying in direct proportion to the effectivelength of the resistance strip 84. The lower portion of the circuitryoperates the same way as that which has been described and it similarlyproduces a voltage on an output line 108 that is proportional to thedistance between the connected end of the resistance element 84 and thepoint of contact that is made by the actuator 94 associated with theother sash door in the track.

Referring to the composite electrical schematic diagram of the circuitryof the fume hood controller, if the separate drawings FIGS. 10a, 10b,10c, 10d and 10e are placed adjacent one another in the manner shown inFIG. 10, the total electrical schematic diagram of the fume hoodcontroller 20 is illustrated. The operation of the circuitry of FIGS.10a through 10e will not be described in detail. The circuitry is drivenby a microprocessor and the important algorithms that carry out thecontrol functions of the controller will be hereinafter described.Referring to FIG. 10c, the circuitry includes a Motorola MC 68HC11microprocessor 120 which is clocked at 8 MHz by a crystal 122. Themicroprocessor 120 has a databus 124 that is connected to a tri-statebuffer 126 (FIG. 10d) which in turn is connected to an electricallyprogrammable read only memory 128 that is also connected to the databus124. The EPROM 128 has address lines A0 through A7 connected to thetri-state buffer 126 and also has address lines A8 through A14 connectedto the microprocessor 120.

The circuitry includes a 3 to 8-bit multiplexer 130, a data latch 132(see FIG. 10d), a digital-to-analog converter 134, which is adapted toprovide the analog outputs indicative of the volume of air beingexhausted by the fume hood, which information is provided to roomcontroller 22 as has been previously described with respect to FIG. 2.Referring to FIG. 10b, an RS232 driver 136 is provided for transmittingand receiving information through the hand held terminal. The circuitryillustrated in FIG. 9 is also shown in the overall schematic diagramsand is in FIGS. 10a and 10b. The other components are well known andtherefore need not be otherwise described.

As previously mentioned, the apparatus utilizes a flow sensor preferablylocated in the exhaust duct 70 to measure the air volume that is beingdrawn through the fume hood. The volume flow rate may be calculated bymeasuring the differential pressure across a multi-point pitot tube orthe like. The preferred embodiment utilizes a differential pressuresensor for measuring the flow through the exhaust duct and the apparatusutilizes control schemes to either maintain the flow through the hood ata predetermined average face velocity, or at a minimum velocity in theevent the fume hood is closed or has a very small by-pass area.

The fume hood controller can be configured for almost all known types offume hoods, including fume hoods having horizontally movable sash doors,vertically movable sash doors or a combination of the two. As can beseen from the illustrations of FIGS. 2 and 10, the fume hood controlleris adapted to control an exhaust damper or a variable speed fan drive,the controller being adapted to output signals that are compatible witheither type of control. The controller is also adapted to receiveinformation defining the physical and operating characteristics of thefume hood and other initializing information. This can be input into thefume hood controller by means of the hand held terminal which ispreferably a lap top computer that can be connected to the operatorpanel 34. It should be appreciated that the day/night operation may beprovided, but is not the preferred embodiment of the system; if it isprovided, the information relating to such day/night operation should beincluded.

Operational information

1. Time of day;

2. Set day and night values for the average face velocity (SVEL), feetper minute or meters per second;

3. Set day and night values for the minimum flow, (MINFLO), in cubicfeet per minute;

4. Set day and night values for high velocity limit (HVEL), F/m orM/sec;

5. Set day and night values for low velocity limit (LVEL), F/m or M/sec;

6. Set day and night values for intermediate high velocity limit (MVEL),F/m or M/sec;

7. Set day and night values for intermediate low velocity limit (IVEL),F/m or M/sec;

8. Set the proportional gain factor (KP), analog output per error inpercent;

9. Set the integral gain factor (KI), analog output multiplied by timein minutes per error in percent;

10. Set derivative gain factor (KD), analog output multiplied by time inminutes per error in percent;

11. Set feed forward gain factor (KF) if a variable speed drive is usedas the control equipment instead of a damper, analog output per CFM;

12. Set time in seconds (DELTIME) the user prefers to have the fullexhaust flow in case the emergency button is activated;

13. Set a preset percent of last exhaust flow (SAFLOQ) the user wishesto have once the emergency switch is activated and DELTIME is expired.

The above information is used to control the mode of operation and tocontrol the limits of flow during the day or night modes of operation.The controller includes programmed instructions to calculate the stepsin paragraphs 3 through 7 in the event such information is not providedby the user. To this end, once the day and night values for the averageface velocity are set, the controller 20 will calculate high velocitylimit at 120% of the average face velocity, the low velocity limit at80% and the intermediate limit at 90%. It should be understood thatthese percentage values may be adjusted, as desired. Other informationthat should be input include the following information which relates tothe physical construction of the fume hood. It should be understood thatsome of the information may not be required for only vertically orhorizontally moveable sash doors, but all of the information may berequired for a combination of the same. The information requiredincludes vertical segments, which is defined to be a height and widthdimension that may be covered by one or more sash doors. If more thanone sash door is provided for each segment, those doors are intended tobe vertically moveable sash doors, analogous to a double sashresidential window. The information to be provided includes thefollowing:

14. Input the number of vertical segments;

15. Input the height of each segment, in inches;

16. Input the width of each segment, in inches;

17. Input the number of tracks per segment;

18. Input the number of horizontal sashes per track;

19. Input the maximum sash height, in inches;

20. Input the sash width, in inches;

21. Input the location of the sash sensor from left edge of sash, ininches;

22. Input the by-pass area per segment, in square inches;

23. Input the minimum face area per segment, in square inches;

24. Input the top lip height above the horizontal sash, in inches.

The fume hood controller 20 is programmed to control the flow of airthrough the fume hood by carrying out a series of instructions, anoverview of which is contained in the flow chart of FIG. 11. Afterstart-up and outputting information to the display and determining thetime of day, the controller 20 reads the initial sash positions of alldoors (block 150), and this information is then used to compute the openface area (block 152). If not previously done, the operator can set theaverage face velocity set point (block 154) and this information is thenused together with the open face area to compute the exhaust flow setpoint (SFLOW) (block 156) that is necessary to provide the predeterminedaverage face velocity given the open area of the fume hood that has beenpreviously measured and calculated. The computed fume hood exhaust setpoint is then compared (block 158) with a preset or required minimumflow, and if the computed set point is less than the minimum flow, thecontroller sets the set point flow at the preset minimum flow (block160). If it is more than the minimum flow, then it is retained (block162) and it is provided to both of the control loops.

If there is a variable speed fan drive for the fume controller, i.e.,several fume hoods are not connected to a common exhaust duct andcontrolled by a damper, then the controller will run a feed-forwardcontrol loop (block 164) which provides a control signal that is sent toa summing junction 166, which control signal represents an open looptype of control action. In this control action, a predicted value of thespeed of the blower is generated based upon the calculated opening ofthe fume hood, and the average face velocity set point. The predictedvalue of the speed of the blower generated will cause the blower motorto rapidly change speed to maintain the average face velocity. It shouldbe understood that while the feed forward functional aspect of thecontrol is always operative, it provides correction principally onlywhen the sash position has been changed and the change producesrelatively large errors. After such corrections have been made, then asecond control loop performs the dominant control action for maintainingthe average face velocity constant in the event that a variable speedblower is used to control the volume of air through the fume hood.

After the sash position has been changed, and the new air volume hasbeen established principally by the operation of the feed forward loop,then the proportional-integral-derivative control loop provides thecontrol and this is accomplished by the set flow signal being providedto block 168 which indicates that the controller computes the error bydetermining the absolute value of the difference between the set flowsignal and the flow signal as measured by the exhaust air flow sensor inthe exhaust duct. Any error that is computed is applied to the controlloop identified as the proportional-integral-derivative control loop(PID) to determine an error signal (block 170) and this error signal iscompared with the prior error signal from the previous sample todetermine if that error is less than a deadband error (block 172). If itis, then the prior error signal is maintained as shown by block 174, butif it is not, then the new error signal is provided to output mode 176and it is applied to the summing junction 166. That summed error is alsocompared with the last output signal and a determination is made if thisis within a deadband range (block 180) which, if it is, results in thelast or previous output being retained (block 182). If it is outside ofthe deadband, then a new output signal is provided to the damper controlor the blower (block 184). In the event that the last output is theoutput as shown in block 182, the controller then reads the measuredflow (MFLOW) (block 186) and the sash positions are then read (block188) and the net open face area is recomputed (block 190) and adetermination made as to whether the new computed area less the oldcomputed area is less than a deadband (block 192) and if it is, then theold area is maintained (block 194) and the error is then computed again(block 168). If the new area less the old area is not within thedeadband, then the controller computes a new exhaust flow set point asshown in block 156.

One of the significant advantages is that the controller is adapted toexecute the control scheme in a repetitive and extremely rapid manner.The exhaust sensor provides flow signal information that is inputted tothe microprocessor at a speed of approximately one sample per 100milliseconds and the control action described in connection with FIG. 11is completed approximately every 100 milliseconds. The sash doorposition signals are sampled by the microprocessor every 200milliseconds. The result of such rapid repetitive sampling and executingof the control actions results in extremely rapid operation of thecontroller. It has been found that movement of the sash will result inadjustment of the air flow so that the average face velocity is achievedwithin a time period of only approximately 3-4 seconds after the sashdoor reposition has been stopped.

In the event that the feed forward control loop is utilized, thesequence of instructions that are carried out to accomplish running ofthis loop is shown in the flow chart of FIG. 12, which has thecontroller using the exhaust flow set point (SFLOW) to compute thecontrol output to a fan drive (block 200), which is identified as signalAO that is computed as an intercept point plus the set flow multipliedby a slope value. The intercept is the value which is a fixed outputvoltage to a fan drive and the slope in the equation correlates exhaustflow rate and output voltage to the fan drive. The controller then readsthe duct velocity (DV) (block 202), takes the last duct velocity sample(block 204) and equates that as the duct velocity value and starts thetiming of the maximum and minimum delay times (block 206) which thecontroller uses to insure whether the duct velocity has reached steadystate or not. The controller determines whether the maximum delay timehas expired (block 208), and if it has, provides the output signal atoutput 210. If the maximum delay has not expired, the controllerdetermines if the absolute value of the difference between the last ductvelocity sample and the current duct velocity sample is less than orequal to a dead band value (block 212). If it is not less than the deadband value, the controller then sets the last duct value as equal to thepresent duct value sample (block 214) and the controller then restartsthe minimum delay timing function (block 216). Once this isaccomplished, the controller again determines whether the maximum delayhas expired (block 208). If the absolute value of the difference betweenthe last duct velocity and the present duct velocity sample is less thanthe dead band, the controller determines whether the minimum delay timehas expired which, if it has as shown from block 218, the output isprovided at 210. If it has not, then it determines if the maximum delayhas expired.

Turning to the proportional-integral-derivative or PID control loop, thecontroller runs the PID loop by carrying out the instructions shown inthe flow chart of FIG. 13. The controller uses the error that iscomputed by block 168 (see FIG. 11) in three separate paths. Withrespect to the upper path, the controller uses the preselectedproportional gain factor (block 220) and that proportional gain factoris used together with the error to calculate the proportional gain(block 222) and the proportional gain is output to a summing junction224.

The controller also uses the error signal and calculates an integralterm (block 226) with the integral term being equal to the priorintegral sum (ISUM) plus the product of loop time and any error and thiscalculation is compared to limits to provide limits on the term. Theterm is then used together with the previously defined integral gainconstant (block 230) and the controller than calculates the integralgain (block 232) which is the integral gain constant multiplied by theintegration sum term. The output is then applied to the summing junction224.

The input error is also used by the controller to calculate a derivativegain factor which is done by the controller using the previously definedderivative gain factor from block 234 which is used together with theerror to calculate the derivative gain (block 236) which is thereciprocal of the time in which it is required to execute the PID loopmultiplied by the derivative gain factor multiplied by the currentsample error minus the previous sample error with this result beingprovided to the summing junction 224.

The control action performed by the controller 20 as illustrated in FIG.13 provides three separate gain factors which provide steady statecorrection of the air flow through the fume hood in a very fast actingmanner. The formation of the output signal from the PID control looptakes into consideration not only the magnitude of the error, but as aresult of the derivative gain segment of control, the rate of change ofthe error is considered and the change in the value of the gain isproportional to the rate of change. Thus, the derivative gain can seehow fast the actual condition is changing and works as an "anticipator"in order to minimize error between the actual and desired condition. Theintegral gain develops a correction signal that is a function of theerror integrated over a period of time, and therefore provides anynecessary correction on a continuous basis to bring the actual conditionto the desired condition. The proper combinations of proportional,integral and derivative gains will make the loop faster and reach thedesired conditions without any overshoot.

A significant advantage of the PID control action is that it willcompensate for perturbations that may be experienced in the laboratoryin which the fume hood may be located in a manner in which othercontrollers do not. A common occurrence in laboratory rooms which have anumber of fume hoods that are connected to a common exhaust manifold,involves the change in the pressure in a fume hood exhaust duct that wascaused by the sash doors being moved in another of the fume hoods thatis connected to the common exhaust manifold. Such pressure variationswill affect the average face velocity of those fume hoods which had nochange in their sash doors. However, the PID control action may adjustthe air flow if the exhaust duct sensor determines a change in thepressure. To a lesser degree, there may be pressure variations producedin the laboratory caused by opening of doors to the laboratory itself,particularly if the differential pressure of the laboratory room ismaintained at a lesser pressure than a reference space such as thecorridor outside the room, for example.

It is necessary to calibrate the feed forward control loop and to thisend, the instructions illustrated in the flow chart of FIG. 14 arecarried out. When the initial calibration is accomplished, it ispreferably done through the hand held terminal that may be connected tothe operator panel via connector 38, for example. The controller thendetermines if the feed forward calibration should perform a correction(block 242) and if it should, then the controller sets the analog outputof the fan drive to a value of 20 percent of the maximum value, which isidentified as value AO1 (block 244). The controller then sets the lastsample duct velocity (LSDV) as the current duct velocity (CDV) (block246) and starts the maximum and minimum timers (block 248).

The controller ensures the steady state duct velocity in the followingway. First by checking whether the max timer has expired, and then, ifthe max timer has not expired, the controller determines if the absolutevalue of the last sample duct velocity minus the current duct velocityis less than or equal to a dead band (block 270), and if it is, thecontroller determines if the min timer has expired (block 272). If ithas not, the controller reads the current duct velocity (block 274). Ifthe absolute value of the last sample duct velocity minus the currentduct velocity is not less than or equal to a dead band (block 270), thenthe last sample duct velocity is set as the current duct velocity (block276) and the min timer is restarred (block 278) and the current ductvelocity is again read (block 274).

In case either the max timer or min timer has expired, the controllerthen checks the last analog output value to the fan drive (252) andinquires whether the last analog output value was 70 percent of themaximum output value (block 254). If it is not, then it sets the analogoutput value to the fan drive at 70 percent of the max value AO2 (block256) and the steady state duct velocity corresponding to AO1. Thecontroller then repeats the procedure of ensuring steady state ductvelocity when analog output is AO2 (block 258). If it is at the 70percent of max value, then the duct velocity corresponds to steady statevelocity of AO2 (block 258). Finally, the controller (block 262)calculates the slope and intercept values.

The result of the calibration process is to determine the duct flow at20% and at 70% of the analog output values, and the measured flowenables the slope and intercept values to be determined so that the feedforward control action will accurately predict the necessary fan speedwhen sash door positions are changed.

The apparatus is adapted to rapidly calculate on a periodic basisseveral times per second, the uncovered or open area of a fume hoodaccess opening that is capable of being covered by one or more sashdoors as previously described. As is shown in FIG. 6, the actuator 94 ispreferably located at the righthand end of each of the horizontallymovable doors of which there are four in number as illustrated. Theposition indicating capability of the switching mechanism 80 provides asignal having a voltage level for each of the four doors which isindicative of the position of the particular sash door along itsassociated track. While the actuators 94 are shown at the righthandportion of the sash doors, it should be understood that they may bealternatively located on the lefthand portion, or they could be locatedat virtually any location on each door, provided that the relationshipbetween the width of the door and the position of the actuator isdetermined and is input into the fume hood controller.

It should be appreciated that having the location of the actuators 94 ata common position, such as the right end, simplifies the calculation ofthe uncovered opening. While the fume hood shown in FIG. 6 is of thetype which has four horizontally movable doors 76 that are housed withina frame structure 78 that itself is vertically movable, the fume hoodcontroller apparatus is adapted to be used with up to four movable sashdoors in a single direction, i.e., horizontally, and a perpendicularlymovable sash door frame. However, there are five analog input ports inthe controller for inputting position information regardless of whetherit is horizontal or vertical and the controller can be configured toaccommodate any combination of horizontally and vertically movable doorsup to a total of five. To this end, it should be appreciated that thereare vertically movable double sash doors in certain commerciallyavailable fume hoods, which configuration is not specifically shown inthe drawings, with the double sash configuration being housed in asingle frame structure that itself may be horizontally movable. The fumehood controller may treat the double sash door configuration in thevertical direction much the same as it operates with the horizontallymovable sash doors that operate in two tracks as shown in FIG. 6.

Turning now to FIG. 15, the flow chart for the fume hood controlleroperation as it calculates the uncovered portion of the opening of thefume hood as illustrated for the embodiment of FIG. 6 with respect tothe four horizontally movable doors. The flow chart operation would alsobe applicable for determining the uncovered area for the embodiment ofFIG. 4 as well. The initial step is to read each sash door position(block 300). The next step is to sort the sash doors to determine thesash door positions relative to the left edge of the opening (block302). It should be understood that the determination could be made fromthe right edge just as easily, but the left edge has conveniently beenchosen. The apparatus then initializes the open area 304 as being equalto zero and then the apparatus computes the distance between the rightedge of the sash door nearest the left edge of the opening and the rightedge of the next sash door that is adjacent to it (block 306).

If the difference between the edges, as determined by the actuatorlocation, is greater than the width of the sash (block 308), then thenet open area is set to be equal to the net open area plus thedifference minus the sash door width (block 310) and this value isstored in memory. If the difference is less than the sash door width,then the program proceeds to repeat for the next two pair of sash doors(block 312) as shown. In either event, then the program similarlyrepeats for the next two pairs of sash doors. After the controllerperforms its repetitions to calculate any open area between all of thesash doors, then the controller checks the distance between the rightedge of the nearest sash door and the left track edge which iscomparable to the left opening (block 314) and if the left difference isless than the sash door width (block 316) the controller then checks thedistances between the left edge of the furthest sash door and the rightedge of the track, i.e., the right opening 318. If the left differenceis not less than the sash door width, then the net open area isdetermined to be equal to the net open area plus any left difference(block 320). The controller then determines if the right difference isless than the sash width (block 322) which, if it is, results in the netface area being equal to the net open area plus the fixed area (block324) with the fixed area being the preprogrammed by-pass area, if any.If the right difference is not less than the sash width, then thecontroller determines that the net open area equals the net open areaplus the right difference (block 326). In this way, the net open area isdetermined to be the addition between any open areas between sash doorsand between the rightward sash door and the right edge of the openingand the difference between the left edge of the leftmost sash door andthe left edge of the opening.

Turning now to FIG. 16, a flow chart of operation of the apparatus fordetermining the uncovered area of the opening for a fume hood which hasmultiple vertically moveable sash doors is shown. The controller, wheninitially configured, requires the input of the width of each segment,the number of such segments, the minimum face area, i.e., the by-passarea, plus any other residual open area with the sash doors closed, andthe number of sash doors per segment (block 330). The controller thensets the area equal to zero (block 332) and begins the calculation forthe first segment (block 334) and sets the old height equal to zero(block 336). It then begins with the first sash door (block 338) andreads the sash position (block 340), inputs the slope and intercept(block 342) from the prior calibration routine, and calculates theheight for that sash door and segment (block 344). The apparatus thendetermines if it is sash door number 1, which if it is, forwards theheight of the segment (block 348), obtains the width of the segment(block 350) and calculates the area by multiplying the height times thewidth (block 358). If the sash door was not the number 1 sash, then thecontroller determines if the height of the segment and sash was lessthan the old height, which if it is, then the height of the segment isset as the height (block 352) and the next sash door is made the subjectof inquiry (block 354) and the old height is retrieved (block 356) andthe controller returns to block 338 to repeat the calculations for theother segments and sash doors. After the sash doors for a segment havebeen considered, and the area of the segment determined (block 358), thecontroller determines if the area for the segment is less than theminimum flow area, and if it is, then the area is set to the minimumflow area (block 362). If it is greater than the minimum flow area, thenthe area for the segment is determined to be equal to the by-pass areaplus the calculated area for the segment (block 364). The area is thencalculated as the prior calculated area plus the area of the segmentunder consideration (block 366), and the controller then proceeds toconsider the next segment (block 368). After all segments have beenconsidered, the total area is obtained (block 370).

The apparatus is also adapted to determine the uncovered area of acombination of vertically and horizontally moveable sash doors, such asthe fume hood illustrated in FIG. 6, which has four horizontallymoveable sash doors that are contained in two sets of tracks, with thesets of tracks being contained in a frame structure which is itselfvertically moveable. As previously mentioned, there is an upper lip 77having a front thickness of about 2 inches, the exact dimension of whichcan vary with the manufacturer's design, a lower portion 79 of the frame78, and a by-pass area 75. As may be appreciated, when the frame 78 isin its lowermost position, the entire by-pass area is "open" and air maybe moved through it. As the frame is raised, the portion of the sashdoors 76 which cover the opening will increasingly cover the by-passarea as shown. In the particular illustration of FIG. 6, thehorizontally moveable doors overlap and are completely closed, but theframe is shown being slightly raised.

To determine the uncovered area of the combination sash door fume hood,the following specific steps are performed. The net open area, i.e., theuncovered area, is the sum of the vertical (hereinafter "V" in theequations) area and the horizontal (hereinafter "H") area:

    Net Open Area=V area+H area

with the horizontal area being determined as follows:

    H area=H width * minimum of {panel Ht; Max of (panel Ht+top lip Ht+min. face Ht-sash Ht; 0)}

with the H width comprising the previously described operation beingperformed with respect to the horizontally movable sash doors. Thevertical area (V area) is determined by the following equation:

    V area=Max of (Sash Ht * V width; minimum face area)

To complete the determination, the Net Face Area is then equal to thesum of the Net Open Area and the Fixed or by-pass Area:

    Net Face Area=Net Open Area+Fixed Area

In accordance with the present invention, and referring to FIG. 17, aflow chart is illustrated which compensates for the presence of a grilleor screen (FIG. 3, grille 65) that can provide resistance to air flowthrough the by-pass opening that is otherwise not blocked by themovement of the door of the fume hood. While the flow chart isillustrated for a fume hood of the type shown in FIG. 3, where only asingle vertically movably door is provided, such a compensation factoris adapted for use with any number of doors that may be moved verticallyand/or horizontally, with such movement varying the effective size ofthe by-pass opening.

In the manner as previously described, the vertical position of the sashis determined at block 372, and this information is forwarded to blocks374 and 376 and summing block 378. The by-pass height data is providedby block 380 and resulting unblocked vertical dimension is determined atblock 382, which provides a limiting minimum value if necessary andprovides that value to a multiplier block 384 which multiplies the inputvalue by a conductance factor from block 386, which ranges fromapproximately 0.1 to 1, and which is empirically determined. Theconductance factor is a function of the resistance to flow that isprovided by the presence of the grille or screen, and is different fordifferent styles of grilles or screens. The conductance factor ispreferably determined by adjusting the value of the same and measuringany deviation of the face velocity from the desired face velocity whileoperating the fume hood with the sash doors in various positions whichchange the size of the unblocked portion of said by-pass area. The valueof the conductance factor is then optimized to reduce any deviation to adesired minimum.

The resulting product of block 384 is an effective by-pass verticalvalue that is applied to summing block 388. The vertical facecalculation from block 374 is also applied to block 388 and the summedvalues are applied to switch logic block 390. If logic block passes thesummed value to its output, then that value is multiplied in block 392by the sash width from block 394 to obtain an effective area and it isadded to any fixed open area provided by block 396 in summing block 398to provide the total face area calculation. The failure detection block376 operates to cause the switch logic block 390 to use the output froma 1/2 multiplier block 400 which has an input value of the maximum openheight from a block 402 when a failure is detected. This valuerepresents a forced value rather than the calculated value from thesumming block 388.

From the foregoing detailed description, it should be appreciated that afume hood controller has been shown and described that has superiorcapabilities in being able to accurately control the flow of air tomaintain either a desired or minimum face velocity, and to compensatefor the presence of a grille or screen or the like that may be locatedto cover a by-pass opening. The compensation for resistance to flowthrough the screen enables accurate control beyond mere calculation ofthe size of the by-pass opening that is not covered by the movement ofthe doors.

While various embodiments of the present invention have been shown anddescribed, it should be understood that various alternatives,substitutions and equivalents can be used, and the present inventionshould only be limited by the claims and equivalents thereof.

Various features of the present invention are set forth in the followingclaims.

What is claimed is:
 1. Apparatus for controlling the air flow through afume hood to maintain a predetermined average face velocity through anuncovered portion of a face opening of a fume hood of the type which hasat least one moveable sash door adapted to cover the opening as the fumehood sash door is moved and having a by-pass opening with a grille meansoverlying the opening, the by-pass opening being adapted to be at leastpartially blocked when at least one of said sash doors is moved touncover said face opening, the fume hood being in communication with anexhaust duct for expelling air and fumes from the fume hood, saidapparatus comprising:means for detecting the position of each moveablesash door and generating a position signal that is indicative of thesash door position; means responsive to said position signals fordetermining the size of the uncovered portion of the face opening; meansresponsive to said position signals for determining the overall size ofthe unblocked portion of said by-pass area, and modifying saiddetermined overall size by a conductance factor to compensate for airflow resistance resulting from said grille means, thereby resulting inan effective size of unblocked portion of said by-pass area; means formeasuring the actual flow of air through the exhaust duct and generatingan actual flow signal that is indicative of the actual flow of airthrough the exhaust duct; modulating means for varying the flow of airthrough the exhaust duct responsive to a control signal being receivedfrom a controller means; controller means responsive to said determinedsize of said uncovered portion of the opening and the effective size ofthe unblocked portion of the by-pass area and said actual flow signalfor controlling the flow modulating means to generate a desired flowrate signal value, said desired flow rate signal corresponding to a flowrate that is sufficient to maintain the predetermined average facevelocity through the uncovered portion of the opening, said controllermeans comparing said desired flow rate signal and said actual flow ratesignal and generating an error signal indicative of any error thatexists, said controller means generating and outputting a control signalto said modulating means for selectively reducing said error signal to apredetermined minimum value.
 2. Apparatus as defined in claim 1 whereinsaid conductance factor is a value greater than 0 and less than
 1. 3.Apparatus as defined in claim 2 wherein said conductance factor isdetermined as a result of adjusting the value of the same and measuringany deviation of the face velocity from the desired face velocity whileoperating the apparatus with the sash doors in various positions whichchange the overall size of the unblocked portion of said by-pass area,and thereafter optimizing the value of said conductance factor to reducesaid deviation to a desired minimum.
 4. Apparatus for controlling a flowcontrol means for controlling the air flow through a fume hood tomaintain a predetermined average face velocity through an uncoveredportion of a face opening of a fume hood of the type which has at leastone moveable sash door adapted to cover the opening as the fume hoodsash door is moved and having a by-pass opening with a grille meansoverlying the by-pass opening, the by-pass opening being adapted to beat least partially blocked responsive to movement of at least one ofsaid sash doors, the fume hood being in communication with an exhaustduct for expelling air and fumes from the fume hood, said apparatuscomprising:means for determining the position of each of said sash doorsand generating signals indicative thereof; processing means includingmemory means for determining the size of the uncovered face openingresponsive to said generated signals and to stored data in said memorymeans relating to the physical and operational parameters of the fumehood, said processing means being adapted to determine the overall sizeof the unblocked by-pass opening responsive to said generated signalsand to said stored data; said processing means modifying the overallsize of said unblocked by-pass opening by a conductance factor toprovide an effective size of said unblocked by-pass opening; saidprocessing means adding said size of the uncovered face opening and saideffective size of said unblocked by-pass opening to provide a totaleffective opening size, and controlling said flow control means toprovide the desired average face velocity utilizing said total effectiveopening size.
 5. Apparatus as defined in claim 4 wherein the flowcontrol means comprises a controller for varying the speed of operationof a variable speed blower.
 6. Apparatus as defined in claim 4 whereinthe flow control means comprises a controller for adjusting the positionof a variable position damper means adapted to control the flow of airin the exhaust duct of the fume hood.
 7. A method for controlling a flowcontrol means for controlling the air flow through a fume hood tomaintain a predetermined average face velocity through an uncoveredportion of a face opening of a fume hood of the type which has at leastone moveable sash door adapted to cover the opening as the fume hoodsash door is moved and having a by-pass opening with a grille meansoverlying the by-pass opening, the by-pass opening being adapted to beat least partially blocked when at least one of said sash doors is movedto uncover said face opening, the fume hood being in communication withan exhaust duct for expelling air and fumes from the fume hood, saidmethod comprising:determining the position of each of said sash doorsand generating signals indicative thereof; determining the size of theuncovered face opening, responsive to said generated signals and tostored data in memory means of a processing means, said data relating tothe physical and operational parameters of the fume hood; determiningthe overall size of the unblocked by-pass opening responsive to saidgenerated signals and to said stored data; applying a conductance factorto the overall size of said unblocked by-pass opening to provide aneffective size of said unblocked by-pass opening; adding said size ofthe uncovered face opening and said effective size of said unblockedby-pass opening to provide a total effective opening size, and,controlling said flow control means to provide the desired average facevelocity utilizing said total effective opening size.
 8. A method asdefined in claim 7 wherein said conductance factor is multiplied by saidoverall size of said unblocked by-pass opening, said conductance factorhaving a value greater than 0 and less than
 1. 9. A method as defined inclaim 8 wherein said conductance factor is determined as a result ofadjusting the value of the same and measuring any deviation of the facevelocity from the desired face velocity while operating the apparatuswith the sash doors in various positions which change the overall sizeof the unblocked portion of said by-pass area, and thereafter optimizingthe value of said conductance factor to reduce said deviation to adesired minimum.
 10. A method for controlling the air flow through afume hood to maintain a predetermined average face velocity through anuncovered portion of a face opening of a fume hood of the type which hasat least one moveable sash door adapted to cover the opening as the fumehood sash door is moved and having a by-pass opening with a grille meansoverlying the opening, the by-pass opening being adapted to be at leastpartially blocked when at least one of said sash doors is moved touncover said face opening, the fume hood being in communication with anexhaust duct for expelling air and fumes from the fume hood, saidapparatus comprising:detecting the position of each moveable sash doorand generating a position signal that is indicative of the sash doorposition; determining the size of the uncovered portion of the faceopening with a processing means having an associated memory meansutilizing said position signals and data relating to physical parametersstored in the memory means; determining the overall size of theunblocked portion of said by-pass area utilizing said position signalsand data relating to physical parameters stored in the memory means, andmodifying said determined overall size by a conductance factor tocompensate for air flow resistance resulting from said grille means,thereby resulting in an effective size of unblocked portion of saidby-pass area; adding said size of the uncovered face opening and theeffective size of said unblocked by-pass opening to provide a totaleffective opening size, and measuring the actual flow of air through theexhaust duct, varying the flow of air through the exhaust ductresponsive to said measured actual flow of air through the exhaust ductand the total effective size to provide the desired average facevelocity.
 11. A method as defined in claim 10 wherein said conductancefactor is a value greater than 0 and less than
 1. 12. A method asdefined in claim 10 wherein said step of modifying said determinedoverall size comprises multiplying said overall size by said conductancefactor.
 13. Apparatus for controlling a flow control means forcontrolling the air flow through a fume hood to maintain a predeterminedaverage face velocity through an uncovered portion of a face opening ofa fume hood of the type which has at least one moveable sash dooradapted to cover the opening as the fume hood sash door is moved andhaving a by-pass opening with a grille means overlying the by-passopening, the by-pass opening being adapted to be at least partiallyblocked responsive to movement of at least one of said sash doors, thefume hood being in communication with an exhaust duct for expelling airand fumes from the fume hood, said apparatus comprising:means forgenerating signals indicative of the position of each sash door; controlmeans for determining the size of the uncovered face opening and theoverall size of the unblocked by-pass opening responsive to saidgenerated signals; said control means modifying the overall size of saidunblocked by-pass opening by a factor to compensate for the change inflow characteristic resulting from the presence of said grille means;said control means generating a signal indicating the total effectiveopening size from said generated signals and from said modified size ofsaid unblocked by-pass opening, and controlling said flow control meansto provide the desired average face velocity utilizing said signalindicating said total effective opening size.
 14. Apparatus forcontrolling a flow control means for controlling the air flow through afume hood to maintain a predetermined average face velocity through anuncovered portion of a face opening of a fume hood of the type which hasat least one moveable sash door adapted to cover the opening as the fumehood sash door is moved and having a by-pass opening with a grill meansoverlying the by-pass opening, the by-pass opening being adapted to beat least partially blocked responsive to movement of at least one ofsaid sash doors, the fume hood being in communication with an exhaustduct for expelling air and fumes from the fume hood, said apparatuscomprising:means for generating signals indicative of the position ofeach sash door and of the size of the unblocked by-pass opening; controlmeans adapted to receive said generated signals and provide acompensated by-pass opening signal that is a function of a changed flowcharacteristic through said by-pass opening due to the presence of saidgrille means; said control means controlling said flow control means toprovide the desired average face velocity utilizing said generatedsignals and said compensated by-pass opening signals.
 15. Apparatus forcontrolling a flow control means for controlling the air flow through afume hood to maintain a predetermined average face velocity through anuncovered portion of a face opening of a fume hood of the type which hasat least one moveable sash door adapted to cover the opening as the fumehood sash door is moved and having a by-pass opening with a grill meansoverlying the by-pass opening, the by-pass opening being adapted to beat least partially blocked responsive to movement of at least one ofsaid sash doors, the fume hood being in communication with an exhaustduct for expelling air and fumes from the fume hood, said apparatuscomprising:means for generating first signals indicative of the positionof each sash door; means for generating a second signal indicative ofthe size of the unblocked by-pass opening; control means adapted toreceive said first and second signals and provide a third signal thatcomprises a compensated by-pass opening signal that is a function of achanged flow characteristic through said by-pass opening due to thepresence of said grille means; said control means controlling said flowcontrol means to provide the desired average face velocity utilizingsaid first and third signals.